Since ultrasonic energy can be used to interrogate vessels at great depth, it is only natural that it should be applied to deeply placed arteries in the abdomen. Early studies suggested that high-grade stenoses of the renal artery could be detected by this approach as long as the peak systolic velocity in the renal artery was normalized by that measured in the abdominal aorta. A retrospective study comparing the peak velocity in the renal artery to that from the adjacent abdominal aorta (the renal aortic ratio) showed that if this value exceeded 3.5, it is likely to be associated with a greater than 60% diameter-reducing stenosis. To test this hypothesis, we used duplex scanning to prospectively evaluate 58 renal arteries in 29 patients in whom arteriograms were available. There were 39 renal arteries with 0% to 59% stenosis, 14 with 60% to 99% stenosis, and five occlusions by angiography. Renal duplex scanning accurately diagnosed 38 of 39, 11 of 14, and four of five of these, respectively, giving a sensitivity of 84%, a specificity of 97%, and a positive predictive value of 94% for the detection of a greater than 60% diameter-reducing stenosis. The overall agreement with angiography was 93%. These data show that renal duplex scanning can be used to diagnose renal artery stenosis in patients with hypertension or renal dysfunction, thus providing a rational basis for the selection of patients for angiography.